/Engine/Source/Runtime/InteractiveToolsFramework/Public/BaseGizmos/GizmoMath.h
Construct two mutually-perpendicular unit vectors BasisAxis1Out and BasisAxis2Out which are orthogonal to PlaneNormal
namespace GizmoMath { void GizmoMath::MakeNormalPlaneBasis ( const FVector & PlaneNormal, FVector & BasisAxis1Out, FVector & BasisAxis2Out ) }